The present invention relates to what are commonly referred to as "aerial devices". This is the term used to generically designate a variety of devices for lifting, positioning and holding useful loads above ground level. They include cranes of various types, which use a boom whose end supports a hook, or equivalent attachment means, which can be raised or lowered by means of cables and by which a load can therefore be lifted.
They also include devices which support a load at the end of the boom, without the additional use of a cable lifting mechanism. For example, a fire hose nozzle can be so supported, or a tree-trimming saw, or a platform for a worker, etc.
In all such devices, it is common that the boom itself also is capable of displacement in various respects. It is typically rotatable in azimuth, it is typically movable in elevation, and it is typically extendable, through the use of plural sections which telescope with respect to one another, and sometimes also through the use of additional extensions known as jibs and flys.
The present invention relates to all the foregoing types of aerial devices.
Moreover, although not limited thereto, the present invention relates particularly to such aerial devices, which are readily mobile, in order that they be conveniently and quickly used in various locations and under varying conditions. Such mobile aerial devices are further frequently characterized by having their own wheels, usually in the form of rubber tires, by means of which they can easily be moved from one work location to another. In addition, many though by no means all such mobile devices also have outriggers which can be deployed while the device is at one particular work location. Such outriggers are intended, when deployed, to take the place of the wheels as the supports for the device while it is being used to perform its lifting or supporting functions. Generally, when resting on outriggers, the device has greater performance capabilities than when resting on its wheels.
It will be apparent, that, in devices of the sort under consideration, there are encountered numerous serious problems pertaining to safety. These safety problems can affect the device itself, the load lifted or supported by the device, its human operator, and even other nearby structures not on board of, or attached to the aerial device itself.
A particular form of the aerial devices under consideration, which is subject to a representative variety of those safety problems, is that which is frequently used in carrying out such tasks as electric power line maintenance and repair. Such power lines are typically strung over considerable distances over a variety of terrain. They are supported at a considerable height above ground level, but this height can vary from one span between supporting pylons to another. It is frequently not practical to deenergize them during routine maintenance operatins, as this would interrupt the supply of electrical power which the line is expected to provide.
The type of aerial device which has been used frequently in these circumstances is one that is self-propelled, and therefore has its own transport wheels. It also has outriggers which can be deployed if desired while it is not in rolling motion. Finally it has a telescoping boom, sometimes with optional attachments for a load-bearing hook and lifting cables, on the one hand, and a work platform or basket on the other hand. Jib and fly may also be provided.
As for the safety problems, these include the danger of tipping of the device, if the safe limits of load are exceeded. They include the danger of bending or even breaking of the boom for the same reason. They include the danger of electric shock if the power line is accidentally contacted. And they include the danger of damage to the power line, again in case of unintentional contact.
It is not desired to create the impression that the present invention resides in recognizing problems and dangers such as those mentioned above. Most, if not all of these problems have been known to those skilled in the art, and various attempts have been made to deal with them. These attempts have met with only limited success, due to various factors, particularly the very complex interaction between what might be called the "geometry" of the aerial device itself, the load, and the location of an external object such as the power line.
Thus it is well known that an aerial device is capable of lifting safely different weights of load depending upon the length of extension of the boom, its angle of elevation, and its angle of azimuth. Moreover, all of these parameters are generally different, if the device is a self-propelled one with outriggers, depending on whether or not the outriggers are deployed.
So-called load tables are generally provided by the manufacturer of a particular aerial device, which provide data concerning the foregoing parameters. These tables could, in theory, be used by the device operator to remain within safe limits, insofar as the load weight is concerned. However, in an actual operating situation, it is burdensome and time-consuming to have to resort to such load tables. Moreover, one ingredient in using those tables, namely the weight of the load, is typically known only with low precision, if at all. Therefore, in practice, these load tables are used only in a limited way and are frequently ignored altogether.
Moreover, even if they were used, and used effectively, they would still be of no help whatsoever in dealing with the danger of contacting external objects, which remains unsolved even if the load-related aspects of aerial device operation are conducted within completely safe limits.